1. Field of the Invention
The present invention relates to a grazing incidence interferometer.
2. Description of the Related Art
Conventionally, a grazing incidence interferometer is known as an interferometer which is capable measuring a measurement surface having large swell. As the grazing incidence interferometer, a common path type is known in which a measuring beam and a reference beam travel along identical optical paths, as disclosed in JP-A-2008-32690.
FIG. 6 is a diagram illustrating the configuration of a grazing incidence interferometer 1D of the common path type disclosed in JP-A-2008-32690. The grazing incidence interferometer 1D is comprised of a beam source section 100D, a prism 200D as a beam splitting part and a beam combining part, and a detecting section 400D. The beam source section 100D includes a laser beam source 101 and lenses 102 and 103. The prism 200D has the functions of the beam splitting part for splitting an incident beam into a measuring beam to be made incident on a measurement surface S and a reference beam serving as a measurement reference and of the beam combining part for combining the reference beam and the measuring beam to obtain a combined beam (interference beam). The detecting section 400D includes a lens 401, an imaging means 402 having a CCD (charge-coupled device), and an unillustrated computing means having a CPU (central processing unit).
The beam emitted from the laser beam source 101 is converted to a collimated beam by the lenses 102 and 103, and is then made incident on the prism 200D. Part of this beam is reflected by a bottom surface 200D1 of the prism 200D to serve as a reference beam, and the remaining part is made emergent to outside the prism 200D and is incident on the measurement surface S to serve as a measuring beam. The measuring beam incident on the measurement surface S is reflected by that measurement surface S, is then incident on the prism 200D, and is combined with the reference beam to form a combined beam. The combined beam forms interference fringes on the imaging means 402 through the lens 401. The interference fringes are imaged by the imaging means 402. The unillustrated computing means performs arithmetic processing on the basis of an interference fringe image picked up by the imaging means 402 to thereby obtain the profile of the measurement surface S.
Next, referring to FIG. 6, a description will be given of the effect exerted on the measurement accuracy by the wave front error of the beam emitted from the laser beam source 101. Besides, the wave front error is caused by various factors, for example, influences of the configurations disposed on the optical path. In FIG. 6, in a case where a proximal end side in the traveling direction of the beam is viewed from a distal end side, the wave front of each beam is represented by an image of R. The orientation of the wave front is inverted each time the beam undergoes one reflection. In the grazing incidence interferometer 1D, the reference beam has undergone one reflection at the bottom surface 200D1 of the prism 200D, and the measuring beam has undergone one reflection at the measurement surface S, so that orientations of the wave fronts of the reference beam and the measuring beam, i.e., combined beam components, are inverted with respect to the orientation of the wave front of the beam emitted from the laser beam source 101, and their orientations become identical. Accordingly, in the interference fringes which are formed from the difference between the wave front of the reference beam and the wave front of the measuring beam, even if a distortion is present in the wave front of the beam emitted from the laser beam source 101, that distortion becomes cancel. For this reason, in the grazing incidence interferometer 1D, the wave front error of the beam emitted from the laser beam source 101 does not affect the measurement accuracy.
As the grazing incidence interferometer, in addition to the above-described common path type, a non-common path type is known in which the reference beam and the measuring beam travel along different optical paths, as shown in JP-A-2008-32690. FIG. 7 is a diagram illustrating the configuration of a grazing incidence interferometer 1E of the non-common path type described in JP-A-2008-32690. Hereafter, those functional parts that are identical to those of the grazing incidence interferometer 1D shown in FIG. 6 will be denoted by the same reference numerals, and a description thereof will be omitted or simplified. In addition, in FIG. 7, the double-sided arrow sign indicates a linearly-polarized beam component parallel to the plane of the drawing, while a double-circle sign indicates a linearly-polarized beam component perpendicular to the plane of the drawing.
The grazing incidence interferometer 1E is comprised of a beam source section 100E, a beam splitting section 200E, a beam splitter 300E as a beam combining part, and a detecting section 400E. The beam source section 100E is configured in the same way as the aforementioned beam source section 100D. The beam splitting section 200E includes a beam splitter 201 and a half-wave plate 202. The detecting section 400E includes a quarter-wave plate 403, a lens 404, a three-split prism 405, polarizing plates 406A to 406C, imaging means 407A to 407C, and a computing means 408. The detecting section 400E is adapted to be able to obtain three interference fringe images each of which has a phase differ from each other, and is so adapted as to be able to attain reduction in analysis time and improvement of vibration resistance.
The beam emitted from the laser beam source 101 is made incident on the beam splitter through the lenses 102 and 103, and is thereby split into a reference beam and a measuring beam. The reference beam is transmitted through the half-wave plate 202, and is then incident on the beam splitter 300E. The measuring beam is incident on the measurement surface S, is reflected at the measurement surface S, and is then incident on the beam splitter 300E. The reference beam and the measuring beam incident on the beam splitter 300E are combined into a combined beam, and is made emergent from the beam splitter 300E. The combined beam emergent from the beam splitter 300E is split into 3 phase-shifted beams by the quarter-wave plate 403, the lens 404, the three-split prism 405, and the polarizing plates 406A to 406C, and the respective beams form interference fringes on the respective imaging means 407A to 407C. The respective interference fringes are imaged by the respective imaging means 407A to 407C. The computing means 408 performs arithmetic processing on the basis of the three interference fringe images picked up by the imaging means 407A to 407C to thereby obtain the profile of the measurement surface S.
In this grazing incidence interferometer 1E, since the reference beam does not undergo reflection, the orientation of the wave front becomes identical to the orientation of the wave front of the beam emitted from the laser beam source 101. Meanwhile, as for the measuring beam, since it has undergone one reflection at the measurement surface S, the orientation of the wave front is inverted with respect to the orientation of the wave front of the beam emitted from the laser beam source 101. Accordingly, since the orientations of the wave fronts of the reference beam and the measuring beam of the combined beam components are mutually inverted, the wave front error of the beam emitted from the laser beam source 101 is not canceled in the interference fringes formed on the imaging means 407A to 407C from the combined beam. For this reason, in this grazing incidence interferometer 1E, the wave front error of the beam emitted from the laser beam source 101 affects the measurement accuracy.
Accordingly, a grazing incidence interferometer of the non-common path type has been developed in which the orientations of the wave fronts of the reference beam and the measuring beam of the combined beam components can be arranged properly (e.g., see FIG. 1 in U.S. Pat. No. 6,249,351). FIG. 8 is a diagram illustrating the configuration of a grazing incidence interferometer 1F of the non-common path type described in U.S. Pat. No. 6,249,351. The grazing incidence interferometer 1F is comprised of a beam source section 100F, a beam splitting section 200F, a diffraction grating 300F as a beam combining part, and a detecting section 400F. The beam source section 100F includes the laser beam source 101 and the lens 104. The beam splitting section 200F includes a diffraction grating 203 and a reference mirror 204. The detecting section 400F includes lenses 409 and 410 and a viewing screen 411.
The beam emitted from the laser beam source 101 is incident on the diffraction grating 203, and is thereby split into a measuring beam and a reference beam. The measuring beam is reflected at the measurement surface S, and is then incident on the diffraction grating 300F. The reference beam is reflected at the measurement surface S, and is incident on the diffraction grating 300F. The reference beam is reflected at the reference mirror 204, and is incident on the diffraction grating F. The measuring beam and the reference beam incident on the diffraction grating 300F are combined into a combined beam, which is emergent from the diffraction grating 300F and then forms interference fringes on the viewing screen 411 through the lenses 409 and 410. Accordingly, these interference fringes are imaged by the imaging means, and are subjected to arithmetic processing by the computing means on the basis of the interference fringe image picked up by the imaging means, thus making it possible to obtain the profile of the measurement surface S.
In this grazing incidence interferometer 1F, since the measuring beam and the reference beam both undergo one reflection, the orientations of the wave fronts can be inverted with respect to the orientation of the wave front of the beam emitted from the laser beam source 101, so that the orientations of the wave fronts of the measuring beam and the reference beam of the combined beam components can be arranged properly. For this reason, the wave front error of the beam emitted from the laser beam source 101 is canceled in the interference fringes formed on the viewing screen 411, thus making it possible to prevent the distortion of that wave front from affecting the measurement accuracy.
Each of the above-described grazing incidence interferometers 1D to 1F has a drawback. The grazing incidence interferometer 1D is capable of properly arranging the orientations of the wave fronts of the measuring beam and the reference beam of the combined beam components, and is able to prevent the wave front error of the beam emitted from the laser beam source 101 from affecting the measurement accuracy.
However, since the grazing incidence interferometer 1D has a geometrical-optical restriction in that the measurement accuracy declines appreciably unless the bottom surface 200D1 of the prism 200D is brought into very close proximity to the measurement surface S until the bottom surface 200D1 substantially contacts the measurement surface S, it is necessary to precisely conduct management of distance between the bottom surface 200D1 of the prism 200D and the measurement surface S, so that there is a problem in that the ease of use is poor. Further, the prism 200D or the measurement surface S is in danger of breakage due to the collision or the contact.
With the grazing incidence interferometer 1E, it is unnecessary to precisely conduct management of distance between, on the one hand, the beam splitting section 200E and the beam splitter 300E and, on the other hand, the measurement surface S, so that the ease of use is excellent. However, since the orientations of the wave fronts of the measuring beam and the reference beam of the combined beam components are unfavorably inversed, there is a problem in that the wave front error of the beam emitted from the laser beam source 101 disadvantageously affects the measurement accuracy.
The grazing incidence interferometer 1F is capable of properly arranging the orientations of the wave fronts of the measuring beam and the reference beam of the combined beam components, and is able to prevent the wave front error of the beam emitted from the laser beam source 101 from affecting the measurement accuracy. In addition, it is unnecessary to precisely conduct management of distance between, on the one hand, the beam splitting section 200F and the diffraction grating 300F and, on the other hand, the measurement surface S, so that the ease of use is excellent. However, with the grazing incidence interferometer 1F, there is a problem in that the configuration becomes disadvantageously special since the reference mirror 204 and the diffraction grating 203 are used.